This invention relates generally to communications and in particular to a system and method for analyzing a transmission line.
Historically, new communication technologies are continually being introduced to improve the ease and rate at which data can be exchanged between remote locations. One factor that must be considered when communicating electronic data is the medium over which the data will travel. This is often referred to as determining the channel quality, line characteristics, line transfer function, insertion loss, or channel impulse response. Numerous different types of conductors are utilized to conduct communication signals. One example medium that is commonly installed throughout the world is twisted pair conductors as are traditionally used to provide telephone service between a central office telephone facility and a residence or business.
The medium must be considered because the medium and its condition can affect the rate at which communication may occur. For example, digital subscriber line (DSL) technology utilizes twisted pair conductors. The rate at which systems using the DSL standards may operate is determined in part by the electrical characteristics of the twisted pair between a transmitting device and a receiving device. The factors that control the rate of communication may include the distance between the receiver and transmitter, presence of bridge taps or load coils, the quality of the twisted pair, the quality of connections to the twisted pair, and the amount of noise that the twisted pair picks up, such as crosstalk noise. As data communication speeds increase, the quality of the line and the presence of line anomalies become of greater importance.
It may be desirable to determine characteristics of the line prior to communicating data so that a data transmission rate may be determined or so that it may be determined if the line is able to support communications under a particular standard. For example, if certain line anomalies exist between a first communication unit and a second communication unit, it is desirable to learn of these anomalies and their effect on communication through the line. Moreover, it is desirable to determine the location of the anomalies so that repair or removal of the anomaly may occur. In the particular case of bridge taps and load coils, service technicians are dispatched to locate and remove the bridge tap or load coil. The dispatch of service technicians is expensive and hence, the less time the service technician must spend locating the line anomaly, the lower the cost of the dispatch. Therefore, the more accurately the anomaly location is identified, the less costly the service dispatch because the technician may more rapidly find and fix the anomaly.
One prior art method of line analysis, such as for evaluating the effects of or identifying the location of line anomalies comprises transmission of a high power pulse on the line. Impedance irregularities in the line cause a reflection or echo when encountered by the pulse. Time information is used to determine the location of the anomaly.
This method of line analysis suffers from numerous disadvantages. One disadvantage arises as a result of the necessary, but undesirable, use of a high power pulse. Transmission of a high power pulse on a line disrupts communication and operation of the other pairs in the binder by creating crosstalk between pairs. Another disadvantage of this prior art method arises because of the available echo processing methods. The in-use pairs in the binder with the line being tested create crosstalk in the line being tested. This limits a system""s ability to detect weak return echoes which translate into a limitation on the ability of prior art pulse system to accurately analyze the distant end of a long line. Yet another drawback associated with the prior art method of high power pulse reflection analysis is the limited platforms available to generate a high power pulse. As a result, pulse test equipment must be implemented as a separate piece of test equipment and may not be an integrated circuit. This increases the cost of testing by requiring a separate piece of test equipment and can make its use inconvenient.
The invention overcomes the disadvantages of the prior art by providing a method for apparatus for sequence time domain reflectometry.
In one embodiment, the invention comprises a line probe signal and method of generating the same for use in determining line characteristics. In one embodiment, the invention comprises a method and apparatus for processing a line probe signal to determine channel characteristics, such as to determine the location and type of one or more line anomalies. Line anomalies may comprise open circuit, short circuit, bridge taps, load coils, moisture on the line, or any other aspect that creates an impedance mismatch.
In one embodiment, a method for performing time domain reflectometry on a communication channel comprises generating a sequence signal and transmitting the sequence signal over a communication channel. In one embodiment the sequence signal has an autocorrelation function, which approximates a Kronecker delta function. The communication channel may comprise any channel such as but not limited to fiber optic cable, coaxial cable, power transmission line, network line Ethernet, twisted pair or any channel capable of conducting data. Next, the system receives one or more reflection signals from the communication channel in response to the transmission of the sequence signal. After receipt of the reflection signal, the system correlates the reflection signal with the sequence signal to generate a correlated signal. Due to the autocorrelation properties of the sequence signal, the correlated signal is a linear combination of the near-end echo and the echoes from one or more anomalies. Next, the system may retrieve a template signal. The template signal corresponds or is representative of the near-end echo in the reflection signal. After retrieving the template signal, the system aligns the template signal and the correlated signal to determine a point of alignment. The point of alignment may comprise when the two signal are most similar. Once aligned, the method subtracts the template signal from the correlated signal to remove near-end echo from the correlated signal. Other aspects of the reflection signal may be removed other than near-end echo. Next, the system measures a time interval between the point of alignment and a subsequent peak in the correlated signal. This reveals the amount of time it took for the signal to propagate to a line anomaly and for the reflection signal to return to the receiver. When the propagation time is determined, the system multiplies the time interval by the rate of propagation of the sequence signal through the communication channel to obtain a distance to a line anomaly. The rate of propagation for an electrical signal through a channel is generally known for different channel mediums. In one embodiment the method further includes dispatching a service technician or other personnel to fix the line anomaly.
In various other configurations or embodiments, the template signal may be measured or created by correlating the reflection from a long cable of the type to be tested and known to be free of anomalies or the template may be derived from a detailed circuit analysis of the transceiver and the line interface. In one embodiment, the sequence signal is transmitted at a power level that does not introduce crosstalk into other communication channels.
In another variation or embodiment, the method of operation also performs a circular rotation of the sequence signal to create a rotated sequence signal and transmits the rotated sequence signal over the communication channel. A rotated reflection signal is received and correlated with the rotated sequence signal that was transmitted to create a rotated reflection signal. This correlated rotated signal is aligned with the correlated signal and combined with the correlated signal to reduce or remove correlation artifacts on the correlated signal.
To realize this method of operation, various different configurations of hardware and/or software may be utilized. In one embodiment, a system performs sequence time domain reflectometry to determine the location of impedance mismatches on a channel being configured to communicate data using a digital subscriber line standard. This embodiment comprises a sequence generator configured to generate a maximal length sequence signal connected to a transmitter that is configured to transmit the sequence signal on a channel. This causes the sequence signal to propagate through the channel, the channel being analyzed to determine the location of impedance mismatches or other line anomalies that may affect data transmission. A receiver is configured to receive one or more reflections that result from the sequence signal encountering impedance mismatches as it propagates through the channel. A correlator connects to the receiver and correlates the received signal, which is comprised of one or more reflections, with the sequence signal to generate a correlated signal, which is the linear combination of the impulse responses of the transmission paths to the one or more anomalies. Also included in this configuration is a processor, other hardware or software, configured to determine the time period between a beginning of the sequence signal transmission as determined from the peak of the near-end echo response and the peak of the echo response from the one or more anomalies. The processor, other hardware or software, is configured to calculate a value corresponding to a channel length between the system and an impedance mismatch.
The invention can be implemented from only one end of the channel, such as when access is possible or convenient to only one end, or when invention may be performed at any point along the channel. The invention may be used to classify the line or channel into a data transmission rate group, a cost of service group, or simply whether or not to use the line for high speed data communication. In one embodiment the distortion of the pulse by the transmission medium may be analyzed to discriminate between various types of anomalies.
In one or more other embodiments, the system may include various other features or aspects. In one embodiment, the system is embodied on a communication device configured to communicate data using a digital subscriber line standard. In one embodiment the sequence generator comprises a tapped delay line.
In one embodiment, the invention utilizes an echo cancellation method of operation to perform time domain reflectometry processing. A method of operation based on this alternative embodiment includes processing a reflection signal resulting from transmission of a sequence of bits over a channel to determine the location of line anomalies. This occurs by providing the generated sequence (not correlated with the transmit sequence) to a prediction module, which, in one embodiment, is comprised of a finite impulse response, adaptive filter. The coefficients of the prediction filter are adapted such that the output of the prediction filter approximates the received reflection sequence when the transmit sequence is applied to the input of the filter. When this adaptive procedure converges, the coefficients of the adaptive filter are an estimate of the linear combination of the near-end echo response and the responses from the one or more anomalies.
This method of operation may further include analyzing the coefficient values when the prediction filter output generally resembles the reflection signal to determine the location of impedance mismatches on the channel. In one particular embodiment the prediction filter comprises a finite impulse response filter. In one embodiment the sequences of bits may comprise a sequence selected from the group of sequences consisting of a maximal length sequence, a Barker code, or a Kasami sequence. It should be noted that comparing may include subtracting the prediction filter output from the reflection signal.
The scope is not limited to only the described combinations but is intended to cover any various combination as might be contemplated after reading the specification and claims. Hence an embodiment may include one feature or element or any combination or number of features or elements.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages are included within this description, are within the scope of the invention, and are protected by the accompanying claims.